Add error constant and separation of error handling and logic

• Except for the normal value, the value to be returned is explicitly indicated through a constant.
• Separated error handling and logic

Comment From: snicoll

@devdynam0507 thanks for the PR, but please stop creating duplicate PRs as it generates unnecessary noise for the 3K+ watchers on this repository. If you need to amend a PR, you can push additional commits to your devdynam0507 branch, as this page hints:

Add more commits by pushing to the devdynam0507 branch on devdynam0507/spring-boot.

Comment From: snicoll

@devdynam0507 thank you for the PR but I think the code is fine as it is.